Dominant controls on catchment water quality dynamics: A Germany-wide analysis using data-driven models

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The availability of clean water is one of the United Nations global goal for sustainable development 2030. In Central Europe and Germany elevated inputs of nutrients to surface water, groundwater and finally to marine environments is still a thread for aquatic ecosystems and water supply. At catchment scale the multitude of interrelated processes and input pathways hinder a mechanistic understanding and a clear cause-effect relationship that is needed for effective management. In the last decades great efforts have been put into the improvement of wastewater treatment to reduce point source inputs of phosphorus and nitrogen into surface waters. However, diffuse inputs from agricultural sources are still important and hard to manage as a result of long transport time lags and legacy effects. Consequently, the effects of measures to reduce nitrogen surplus from agriculture on the downstream water quality are often hard to predict. This proposal utilizes a unique dataset of water quality and water quantity across Germany. This data is based on the regular water quality monitoring of Germany's federal states and was compiled by the UFZ. By means of a data-driven analysis, patterns in the concentration and concentration-discharge relationships are used to conclude on dominant processes and input pathways at catchment scale. The proposal focusses on three aims: (a) The classification of the catchments with regards to nutrient export regimes for the last years (younger than 2010). (b) The temporal development of nitrogen:phosphorus ratios for longer time series with changing input pathways. (c) The long-term trajectories of nutrient exports from catchments and the possible development towards chemostatic conditions. The innovative data-driven methods and the unique dataset allow a new view of dominant controls of nutrient exports in the Central-European settings of anthropogenically modified landscapes. The results will be a starting point for complexity reduced water quality model at catchment scale. Moreover, results will be of interest to a broader range of disciplines such as aquatic ecology and environmental management.